2. Shape Effect of Ceria in Gold-Ceria Catalyst for Carbon Monoxide Oxidation Reaction
2.2. Experimental Methods
2.2.1 Preparation of CeO2 Nanocrystals
CeO2 cubes were synthesized according to the reported hydrothermal method.46-47 The 9.6 g of NaOH dissolved in 40 mL of deionized (DI) water was placed in a Teflon cup, followed by missing with 0.868 g of Ce(NO3)3∙6H2O under vigorous stirring for 30 min to form a white slurry. The Teflon cup was placed inside a stainless steel autoclave reactor and kept in an oven at 453 K for 24 h. After the reaction, the solid product was separated by centrifugation with excess deionized water and washed with ethanol. The CeO2 octahedra were also synthesized using the hydrothermal method with some modification.48 The 0.0038 g of Na3PO4‧12H2O dissolved in 40 mL of DI water was load in the Teflon cup, followed by mixing with 0.434 g of Ce(NO3)3∙6H2O by stirring for 30 min. The hydrothermal reaction occurred in an oven at 473 K for 20 h. The precipitates were separated by centrifugation with DI water and washed with ethanol. The resulting CeO2 powders were dried at 353 K for 8 h and calcined at 673 K for 4 h at a heating rate of 5 K/min.
2.2.2 Preparation of Au/CeO2 Catalysts
The microwave-assisted reduction method was used to accelerate the homogeneous nucleation and deposition of Au NPs on CeO2.49 Typically, 40 mg of CeO2 powder with controlled shapes was dissolved in 40 ml of ethylene glycol, and the mixture solution was sonicated for 1 h for complete mixing. After adding 1 mg of HAuCl4·xH2O and 60 µL of oleylamine to the CeO2 solution, the mixture was transferred to the Teflon vessel and placed in the microwave reactor (MARS, CEM Corporation).
The reaction temperature was monitored using an optic probe type thermometer during the microwave- assisted reaction. At 800 W of the power (2.45 GHz) for 30 s, reduction occurred by producing Au/CeO2
catalysts. The resulting catalysts were obtained by centrifugation and re-dispersed in ethanol.
2.2.3 Experimental Characterization
Transmission electron microscopy (TEM) was performed using a JEM-1400 instrument and a JEM-2100F (JEOL) instrument with acceleration voltages of 120 kV and 200 kV, respectively. Energy- dispersive X-ray spectroscopy (EDS) was used for elemental analysis (Oxford instrument, X-Max 80T).
Inductively coupled plasma-optical emission spectrometry (ICP-OES) using a 700-ES model instrument (Varian) was used to determine the amount of Au. Before ICP-OES measurements, the concentration of Au was calibrated using a standard Au solution (Sigma-Aldrich), and the catalyst was dissolved in aqua regia and diluted with water. X-ray photoelectron spectroscopy (XPS) analysis was conducted using K-alpha radiation; an Escalab 250Xi instrument (ThermoFisher) with an Al Kα X-ray radiation source. The spectra were obtained under 3 × 10-8 mbar of pressure with a turbo molecular pump. A flood gun was used to compensate for the charging effect. X-ray diffraction (XRD) patterns were collected by a powder X-ray diffractometer PANalytical X’Pert Pro instrument (Philips) using a Cu Kα X-ray radiation source operating at 40 kV and 30 mA. N2 adsorption experiments were conducted using a BELSORP-max model to measure the BET surface area. Before analysis, the catalysts were pretreated in a vacuum at 423 K for 12 h. Temperature-programmed reduction (TPR) was performed with a thermal conductivity detector (TCD) connected to a gas chromatograph (GC, Agilent 7820A) with a mixed flow of H2/N2.
2.2.4 Catalytic CO Oxidation Activity of Au/CeO2
Catalytic CO oxidation was carried out in a batch reactor. Thin films of the Au/CeO2 catalysts were prepared by drop-casting onto a silicon wafer and treatment with a UV lamp to remove surfactants.
Two mercury (Hg) lamps emitting photons of 184 and 254 nm were used to irradiate the NPs for 1 h.
UV treatment successfully removed only the organic molecules, such as oleylamine, without damaging the NPs, and the organic surfactants were photothermally decomposed by UV light at 184 and 254 nm while open to atmosphere by generating an organic-free surface on the NPs. The batch reactor was maintained under a vacuum up to 1 × 10-8 Torr (1 × 10-11 bar) using rotary and turbo pumps. The gases were introduced into the reactor containing 0.05 bar (40 Torr) of CO and 0.13 bar (100 Torr) of O2
balanced with He (0.83 bar, 620 Torr). All gases were circulated using a circulating pump at a rate of 5.5 L/min, and equilibrium was established after 30 min of recirculation. The reactants and products were detected by GC (YL-6500) equipped with TCD with a Carboxen 1000 column (Supelco). The reaction was conducted at 473–513 K, and the CO conversion was determined by the converted amount of CO to CO2 per unit catalyst weight determined by ICP-OES. The reaction rate was maintained below 20%, which corresponded to the initial reaction rate in a kinetically controlled regime.50 To obtain a rate map according to the CO partial pressure and temperature, the CO partial pressure was adjusted from 0.05 to 0.20 bar, and the reaction temperature was varied from 400 to 600 K using a boron-nitride heater.
To calculate a turnover frequencies (TOF), the concentration of the Au NPs was measured by ICP-OES, and the surface area of Au was calculated according to the average diameter of Au NPs (3 nm) in the experiment. The TOF was determined by the number of moles of CO2 converted per gram of catalyst.
The CO2 partial pressure was calculated as the integral of the areas indicated by GC and considering the reaction temperature and the volume (1 L) inside the reactor.
2.2.5 Density Functional Theory Calculations
We constructed a consistent model of Au NPs supported on CeO2(111) and CeO2(100) surfaces using a two-layered Au9 NP and defect-free CeO2(111) 5×5×2 and CeO2(100) 3×3×2 slab models.40 Based on Jenkins and coworkers’ DFT calculations on a stable structure of Au clusters on CeO2(111),51 we used a two-layered Au9 NP, and its overall morphology was the same on CeO2(100) and CeO2(111) during CO oxidation. The bottom trilayer of the ceria was fixed during the geometric optimization.
Details of the Au/CeO2 model construction and reliability test results are available elsewhere.40, 43-44 The ∆G of sequential CO adsorption on the Au9 NP was calculated as follows:
ΔG = E n∙CO+Au9/CeO2 E Au9/CeO2 n E CO +ΔμCO
where E(system) and n are the DFT-estimated total energy of a corresponding system and the number of adsorbed CO molecules, respectively. The chemical potential of CO and ΔμCO , the chemical potential difference, is given by
μCO T, p = ΔH 0K, p0→T, p0 TΔS 0K, p0→T, p0 + kT ln p p0 ΔμCO μCO T, p E CO
where p0 is set to 1.013 bar and μCO 0K, p0 E(CO). Tabulated temperature-dependent enthalpy and entropy values of CO were adopted from the NIST chemistry web-book52 and NIST-JANAF thermochemical tables.53
Microkinetic modeling was performed to construct a rate map of CO oxidation from the DFT calculation results. We estimated the entropic contribution to the ∆G of CO adsorption by the experimentally verified Campbell’s model.54 The following linear relationship between the entropy of a gas-phase molecule and an adsorbed molecule was applied:
Sadsorbed molecule0 = 0.7 Sgas-phase molecule0 3.3 R . Details regarding the microkinetic modeling are available elsewhere41.
We performed spin-polarized DFT calculations with the Vienna Ab-initio Simulation Package (VASP)55 and the PW91functional.56 To treat the highly localized Ce 4f orbital, DFT+U57 with Ueff = 4.5 eV was applied.58-59 The interaction between the ionic cores and the valence electrons was described using the projector-augmented wave method.60 Valence electron functions were extended with the plane-wave basis to an energy cutoff of 400 eV. The Brillouin zone was sampled at the -point. The
convergence criteria for the electronic structure and geometry were set to 10−3 eV and 0.01 eV/A, respectively. We used a Gaussian smearing function with a finite temperature width of 0.05 eV to improve the convergence of states near the Fermi level. The location and energy of transition states (TSs) were calculated with the climbing-image nudged elastic band method.61-62
Figure 2.1. DFT-calculated ∆G of multiple CO adsorption on (a) Au/CeO2(100) and (b) Au/CeO2(111).
nCO denotes the Au/CeO2 model with n adsorbed CO molecules. Under practical CO oxidation conditions (p(O2) = 0.21 bar and 0.02 bar ≤ p(CO) ≤ 0.20 bar), 8 CO molecules can be stabilized on the Au NP of Au/CeO2(100), whereas 4 CO molecules can be stabilized on the Au NPs of Au/CeO2(111).